1. Field of the Invention
The present invention relates to an air fuel ratio control system for an internal combustion engine, wherein the engine operates, in accordance with the load of the engine, between a combustible mixture of lean air fuel ratio and a combustible mixture of theoretical or rich air fuel ratio mixture.
2. Description of Related Art
A so-called "lean burn internal combustion engine" is known, where the engine, under low load conditions, is operated by supplying a lean air fuel mixture having an air fuel ratio as high as, for example, 20.0. In such a known internal combustion engine, under high engine load conditions, where high engine output power is required, the engine is supplied with an air fuel mixture having a theoretical air fuel ratio or a rich air fuel mixture having an air fuel ratio with a value smaller than the theoretical air fuel ratio. Under low load conditions, where the engine is supplied by a lean air fuel ratio that is higher than a theoretical air fuel ratio, a basic fuel injection amount is first calculated to be an amount of fuel capable of obtaining the theoretical air fuel ratio under particular engine operating conditions determined by a combination of an engine speed value and an intake pressure value or a ratio of the intake air amount to the engine speed, and a lean correction factor (&lt;1.0) is multiplied to the calculated basic injected fuel amount to obtain a final fuel amount to be injected to an intake system of the internal combustion engine from respective injectors. Correcting the basic injected fuel amount by multiplying the lean correction factor by the basic fuel amount produce a lean air fuel mixture that is suitable under particular engine operating conditions.
A determination of the value of the lean correction factor for obtaining an ultra lean air fuel mixture in accordance with the value of the intake pressure and generating no torque increase can be obtained irrespective of a depression of the accelerator pedal to a degree of throttle valve opening larger than a predetermined value, where the value of the intake pressure is substantially maintained and unchanged irrespective of depression of the accelerator pedal. In order to obtain a necessary increase in engine torque upon depression of the accelerator pedal, the air fuel ratio of the air fuel mixture introduced into the engine is changed from a lean air fuel ratio value to a theoretical air fuel ratio. Such a change in the air fuel ratio from a lean air fuel ratio (a large air fuel ratio value) to a theoretical air fuel ratio causes the engine torque to abruptly increase, thereby generating a shock in the engine. In order to prevent such a rapid increase in torque, a second map is made for calculating a lean correction factor based on a combination of throttle valve opening degree values and engine rotational speed. This second map is used for calculating the lean correction factor at an engine load area operating with an intermediate lean air fuel mixture, which is located between a value of the engine load above, with the value of the intake pressure not substantially varying irrespective of depression of the accelerator pedal and another engine load value, above which the engine operates with a theoretical air fuel ratio. The setting of the air-fuel ratio at this intermediate lean area is, for example, between about 16.0 and 18.0. See Japanese Un-Examined Patent Publication (kokai) No. 3-24244.
In the prior art, under low engine load conditions, an ultra lean air fuel mixture of an air fuel ratio as high as, for example, 18.0 to 20.0 is obtained for reducing the amount of nitrogen oxide emission in the exhaust gas. Under middle load conditions, an intermediate lean air fuel mixture of an air fuel ratio in a range, for example, between about 16.0 and 18.0 is obtained in response to a slight increase in engine output power. Under high load conditions, the engine is operated under a theoretical air fuel ratio or an air fuel ratio slightly richer than the theoretical air-fuel ratio. The provision of a medium lean air fuel ratio area between the ultra lean air fuel ratio area and the rich air fuel ratio area permits a smooth increase in engine output torque along the entire load range of the engine, while preventing the occurrence of shock caused by acceleration of the vehicle.
However, the intermediate lean air fuel ratio can generate a large amount of nitrogen oxide in the exhaust gas, and as a result, when the engine is operating in a mode such that it remains with in the intermediate air fuel ratio zone, there is a large amount of nitrogen oxide emission in the exhaust gas which is not suitable in view of recent environmental restrictions concerning emission of nitrogen gas in the exhaust gas of internal combustion engines.